The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a switchable RF coil assembly and an end coil configuration together with a method to minimize the mutual inductance and occurrences of wrap-around artifacts in a whole-body imaging coil array
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed using gradient coils. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The transmission of a radio frequency (RF) excitation pulse through a subject and the reception of the resulting resonant signal is known in the art of nuclear magnetic resonance imaging. Examples of structures capable of transmitting and receiving RF pulses include a helical coil, saddle coil, resonant cavity, and a birdcage resonator or coil. While the use of these structures for transmission and reception of image signals has greatly improved reconstruction of an image, there are some drawbacks to these current designs. For example, such designs can result in the occurrence of what is commonly referred to as wrap-around artifacts that can create distortion during the image reconstruction process. That is, due to non-linear characteristics and an inhomogeneous background magnetic field B0, there are areas outside the FOV that resonate at the same frequency as portions inside the FOV, such that these aliasing wrap-around artifacts can appear in the reconstructed image. These undesirable wrap-around artifacts cause a portion of the imaged subject, which is located outside the FOV, to appear inside of the FOV as part of the volume imaged.
The standard birdcage resonator and other known structures used for whole body imaging have limitations as to the strengths of magnetic fields introduced in the system because of their whole-body imaging methodology. Whole-body coils cause more irradiation of patient volume than equivalent shorter length coils. Irradiation levels are regulated according to an average specific power absorption rate (SAR) per unit mass for patients under examination. As a result, there is a need for an apparatus capable of operating with increased magnetic field strength without exceeding regulated absorption rates. The apparatus should also minimize mutual inductances within the coil configuration and reduce occurrences of wrap-around artifacts during image reconstruction.
It would therefore be desirable to have a switchable FOV coil configuration to restrict sensitivity in areas outside a FOV to reduce occurrences of wrap-around artifacts as compared to standard whole-body coil imaging devices and processes while minimizing the mutual inductance between coils of the configuration.
The present invention provides a switchable FOV magnetic resonance imaging coil configuration and method solving the aforementioned drawbacks.
The invention includes the use of a magnet to produce a magnetic field for MRI imaging of a patient. After the patient is placed within the bore of a magnet having a uniform static magnetic field such that nuclei within the patient are aligned, then excited and encoded using a set of linear magnetic field gradient coils, a FOV is selected by an operator and input into the computer. The computer then transmits signals for activation of a central RF coil only or the central RF coil in combination with first and second RF end coils to perform an imaging scan of the patient. During activation of the center RF coil only, the effective longitudinal length of the RF coil is less than a typical whole-body coil, thereby reducing wrap-around artifacts as compared to the standard length whole-body coil. A shorter RF coil causes limited excitation of nuclei in areas outside of the desired FOV. During activation of the coils in combination, the first and second RF end coils are aligned to form a pair of end coils reacting like a single resonator as opposed to independent end coils placed a certain distance apart.
In accordance with one aspect of the present invention, a switchable FOV coil configuration includes first and second RF coils aligned along a first axis. The second RF coil is coupled to the first RF coil to form an end coil configuration. This end coil configuration has at least one pair of RF coils, but can include more. The RF coil configuration is capable of switching between differing FOV sizes. The switchable FOV coil configuration also includes a central RF coil having a length along the first axis and positioned within the end coil configuration, preferably with some overlap. With activation of both the central RF coil and the end coil configuration, an imaging scan can be acquired that is comparable to a standard whole-body imaging scan.
In accordance with another aspect of the present invention, an MRI apparatus to acquire images is disclosed having an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field B0 through a patient under examination. An RF transceiver system in an RF switch controlled by a pulse module is included to transmit RF signals to an RF coil assembly having a center coil and at least two pairs of interconnected semi-cylindrical coils to acquire magnetic resonance (MR) images of the patient. The MRI apparatus also includes a computer programmed to acquire a desired FOV size for imaging and activates a number of coils of the RF coil assembly in response to the desired FOV size. The computer also acquires data from the number of coils activated and reconstructs an image from the data acquired within the desired FOV
In accordance with yet another aspect of the present invention, a technique of reducing mutual inductance in a switchable FOV MRI device includes the step of providing a first saddle coil and a second saddle coil having a common axis, wherein the first and second saddle coils are rotated relative to one another. Another step performed is positioning a center coil amid the first and second saddle coils. The technique also includes the step of providing a control connected to the center coil and the first and second saddle coils, wherein the control is configured to activate one of the center coil only, or the center coil and the first and second saddle coils simultaneously according to the desired FOV selected for imaging.
The invention also includes an RF end coil configuration that has first and second cylindrical coil elements electrically connected to one another and spaced a distance apart. Each cylindrical coil element has an upper and lower transceiver portion arranged opposite one another to form a center opening and are connected with a set of cross-over leads. The first and second cylindrical coils are positioned parallel to one another such that the center openings are in alignment to accommodate whole-body imaging. Preferably, a third coil element is fitted between the first and second coil elements of the RF end coil configuration and partially overlap the first and second cylindrical coil elements.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.